This disclosure relates to air separation systems for aircraft, and more specifically to an oxygen sensor for a nitrogen generation system.
Aircraft fuel tanks and containers can contain potentially combustible combinations of oxygen, fuel vapors, and ignition sources. In order to prevent combustion, the ullage of fuel tanks and containers is filled with air with high nitrogen concentration, or nitrogen-enriched air (NEA). A nitrogen generation system (NGS) is commonly used to produce NEA for inerting fuel tanks and containers. An air separation module (ASM) in the NGS separates ambient air into NEA, which is directed to fuel tanks and containers, and oxygen-enriched air (OEA), which is rejected overboard. For a given system, the amount of oxygen in the NEA depends on various parameters such as feed flow and operating pressure. Therefore, an oxygen sensor can be used to ensure that the oxygen concentration in the NEA remains below a prescribed level in order to reduce risk of explosion in fuel tanks and containers.
However, oxygen sensors can be susceptible to premature failure due to contamination with siloxane compounds. Siloxane compounds can form silicon dioxide films on top of an oxygen sensor that prevent oxygen from diffusing through the layers of the sensor. As a result, the sensor does not accurately measure the oxygen concentration in the NEA produced by the ASM and requires replacement. Siloxane compounds can be released by various types of sealants used in aircraft. Siloxane compounds are also used in various personal care products, such as shampoos and deodorants, so siloxane compounds can be abundant in ambient air present in aircraft, particularly in aircraft cabins. Even small amounts of siloxane compounds (on the level of parts per billion) can affect the performance of an oxygen sensor.